The present invention relates to binuclear DNA-threading transition metal complex.
The use of nucleic-acid binding compounds for pharmaceutical or diagnostic purposes is widespread and goes back many decades in the history of science. Alkylating, strongly binding compounds belonging to this group have for a long time been used for cytostatic treatment. Less strongly binding antibiotics that function by nucleic-acid binding include numerous dyes, some of which are used today as fluorescent markers for DNA, but which originally were developed as drugs against various parasite diseases. Many of these original drugs bind to DNA by intercalating a large planar aromatic entity between two adjacent basepairs of the DNA, whereas some bind into the minor groove of the DNA.
The probably most well-known metal-based drug is the cis-platin compound invented by Rosenberg, which is based on [cis-Pt(II)Cl2(NH3)2]. It is believed to interact with DNA in a hydrolysed bis-aquo cationic form, which is intercalated between base-pairs and thereafter to bind to DNA bases by co-ordination to the central metal ion.
Another class of DNA-binding compounds are substitution inert transition metal complexes with an octahedral co-ordination sphere, such as [Ru(II) (1,10-phenanthroline)3]2+, whose DNA adducts are stabilised both by electrostatic attraction to the negative deoxyribophosphate oxygens and by hydrophobic effects due to their lipophilic chelate rings. Due to the octahedral co-ordination, the tris-chelates span all the three spatial dimensions and are inherently chiral, in contrast to the square planar co-ordination complexes of Pt(II) or Pd(II). An interesting possibility, to use the propeller-shaped arrangement of ligands around the central metal to obtain stereochemical recognition of nucleic acid binding sites, has up until the present invention not been very successful, probably because the contact area between complex and nucleic acid is too small.
A DNA-binding drug with two functional groups is in itself not a novel idea. For example, several well known drugs used in cancer therapy consist of one bulky sugar group and one planar polycyclic aromatic group, that is generally believed to be intercalated. Also the xe2x80x9cthreadingxe2x80x9d phenomenon, i.e. the fact that a ligand can bind by penetrating through the DNA double helix, having bulky groups in both grooves, is known for several organic-chemical compounds, one example being the antibiotic nogalamycin.
Binuclear metal complexes of Pt(II) and Pd(II) have been described, e.g. in EP-A-0 320 960 and WO 91/03482, but, due to the square planar co-ordination geometry, they lack the stereochemical recognition elements of octahedrally co-ordinated tris-chelates. Also binuclear metal complexes of Pt(IV) which accordingly have an octahedral co-ordination sphere have earlier been described, see e.g. WO 95/07693, WO 88/00947, and WO 84/04524; however, those complexes also lack these stereochemical recognition elements since each Pt(IV) is co-ordinated with at most one substitution-inert bidentate chelating ligand. Platinum based drugs are often associated with problems due to their nephrotoxicity. Non-ionic complexes of Pt(II) have frequently low solubility, and Pt(IV) complexes are likely to be readily reduced to the divalent oxidation state by cell constituents, which in fact may actually be advantageous for activity Without being bound by any mechanism or theory, it is considered likely that chemically inert octahedral complexes with at least one polycyclic heteroaromatic ligand, exemplified by [Ru(1,10-phenanthroline)2 dipyrido[3,2-a:2xe2x80x2,3xe2x80x2-c]phenazine]2+, act by a completely different mechanism from the platinum based drugs, and that this action is excarted by the intact complex interacting non-covalently with the nucleic acid, i.e. intercalation into DNA (see e.g. Hiort, C. et al, JACS 115 (1993) 348).
The present invention relates to a binuclear metal complex, in which one of the metals (M1) have octahedral co-ordination and forms substitution-inert co-ordination bonds to the D1 moiety of the bridging ligand and to a second, at least bidentate, ancillary ligand. This gives rise to a chemically stable, chiral as well as sterically demanding and rigid structure at M1, and since D1 is polycyclic, preferably at least tetracyclic, make threading intercalation a likely binding motif to DNA. The link B of the bridging ligand permits enough flexibility that the other metal M2 can approach to and interact with the DNA also when D1 is intercalated.
The binuclear complexes according to the invention comprises stereochemical recognition elements and thus exhibit better recognition properties than the known mononuclear compounds. In addition, a large advantage in comparison with the known compounds is the larger charge and contact area that substantially increases the DNA affinity of these compounds, exploiting both intercalation and groove modes of binding.
Another advantage of the binuclear metal complexes according to the invention is that they have two rigid chiral centra that are connected via a bridge that is rigidly tied to DNA by intercalating groups. The known binuclear Pt or Pd compounds lack these rigid chiral centra.
There are several other advantage of the substitution inert binuclear transition metal complexes according to the invention compared to the known substitution-inert octahedral transition metal complexes, e.g.:
(1) The compounds according to the invention show stronger binding affinity to nucleic acids than known mononuclear compounds,
(2) The compounds according to the invention show a better stereoselection of specific nucleic targets than known mononuclear compounds,
(3) The threading mechanism used by the compounds according to the invention makes both final association and dissociation processes extremely slow. However, an initial fast association leading to a non-threading complex will first occur, which car have unique selection properties. In this way the selection may be controlled not only by thermodynamic features (the final threading geometry) but also by precursory complexes. The applicants call this novel principle of recognition xe2x80x9ckinetic recognitionxe2x80x9d.
(4) By filling both of the DNA grooves the binuclear complexes according to the invention will have optimal possibilities to interfere with protein-DNA interaction, for example, for inhibition of transcription or replication. This effect is enhanced due to the very slow dissociation of the threading binuclear complex according to the invention.
(5) By a suitable choice of ligands around the metal centra, the compounds according to the invention can be designed to react covalently with DNA a certain period of time after it has reached its target. In this way recognition properties may be combined with the properties of the covalently reacting tumour drugs.
(6) The slow dissociation and bifunctional nature of the binuclear complexes enhance the efficiency of photo-chemical reactions with the DNA target, e.g. photo-cleavage in the case of Rh or Ru complexes.
Furthermore, the high chemical stability of the binuclear complexes exemplified in the invention together with their increased binding affinity and extremely slow dissociation kinetics are likely to make them useful also in cases of Pt-resistant tumours and when the toxicity of platinum agents limits an effective treatment.